Page 224 - Partition & Adsorption of Organic Contaminants in Environmental Systems

BACKGROUND IN PLANT-UPTAKE STUDIES 215

nants may be treated as a series of partitions between plant water and plant
organic constituents, similar to that in fish uptake, where the partition into fish
lipids is largely responsible for bioconcentration of relatively water-insoluble
compounds. However, the plant-uptake system differs from fish-uptake
systems in two important respects: (1) the plant uptake is not restricted to that
from external water (including soil water), since the plant is also exposed to
atmosphere, although water is commonly the transport medium; and (2) the
rate of water transport into plants, by which contaminants enter plants, is
usually more limited than the high rate of water transport into fish. To the first
effect, the atmosphere may act as a medium for both contaminant transport
and dissipation. The second effect leads to the expectation that the contami-
nant level in plants at a given time may deviate profoundly from the equilib-
rium value with external water. In this chapter, we consider only the passive
plant uptake from external water and soil water.

8.2 BACKGROUND IN PLANT-UPTAKE STUDIES

In early studies, Lichtenstein (1959) found that lindane in soil was taken up
by root crops (e.g., carrots and potatoes) more readily from light mineral soils
than from a muck soil. Similarly, Walker (1972) showed that the concentra-
tions of atrazine in shoots of wheat plants growing in 12 different soils were
inversely proportional to soil-organic-matter (SOM) contents. In a more spe-
cific study on the effect of soil type on crop uptake, Harris and Sans (1967)
compared the levels of dieldrin accumulated by carrots, radishes, and other
root crops from three well-characterized contaminated field plots in relation
to the soil pesticide levels; the three soil types studied—a sandy soil, a clay
loam, and a muck soil—differed widely in SOM content (1.4 to 66.5%) and
other soil constituents. Plant dieldrin concentrations were much lower for
crops from the muck soil than from sandy and clay soils; by contrast, soil diel-
drin concentrations were considerably higher in the muck soil than in the two
other soils.
For plant uptake of contaminants from soil-free nutrient solutions, Briggs
et al. (1982) measured the uptake by barley roots of two series of organic
compounds, O-methylcarbamoyloximes and substituted ureas, which vary
widely in lipophilicity. They concluded that the root uptake of both types of
compounds approached the equilibrium values in a relatively short time (24
to 48h). However, the root concentration factors (RCFs), that is, the ratios of
chemical concentrations in roots and in water, increased monotonically, but
not proportionally, with the K ow values of the compounds. Similar empirical
correlations for contaminants in plant roots and leaves were also observed
(Trapp, 1995). In view of the influences of soil type and contaminant identity
on plant uptake, we seek to relate the plant contaminant levels to physico-
chemical properties of the contaminants and to the properties and composi-
tions of plants and soils.